Beginnings of AstronomyKey points: Why ancient humans pursued astronomy; apparent motions of stars and planets on the sky; examples of major astronomical monuments
Knowing the time of year relative to the seasons is critical to determine when to plant crops. It was obvious that the month was about 30 days and the year was about 365 days or 12 months, but none of these relations is exact so any counting scheme had to be "reset" so it wouldnt drift.
The first observatories we know addressed this problem
|Stonehenge is a famous example. Among other things, it was used to identify the first day of summer, the "summer solstice." Ancient astronomers could use it like a giant gunsight to determine when the sun was rising in a certain direction. (From Getty Trust, http://www.getty.edu/artsednet/images/I/stone2.html)|
|This picture gives you an idea of how it might have looked in its prime. It is of a reconstructed Druid ceremony at Stonehenge (From C. Witcombe http://witcombe.sbc.edu/earthmysteries/EMStonehengeB.html)|
View toward "heelstone"
Sunrise at the summer solstice lined up over this prominent stone (from C. Witcombe, http://witcombe.sbc.edu/earthmysteries/EMStonehenge.html)
An even earlier example can be found at Newgrange, just north of Dublin
Through such observations, the ancient astronomers developed calendars that eventually evolved into the one we use todayEarly man also knew that what stars could be seen and how they appeared to move during the night depended on where he was on the earth:
The further north one travels, the higher the North star appears in the sky and stars appear to move closer to parallel to the horizon.
|Here is how the motions look from Tucson (assuming we are looking south and have super-wide-angle eyes that can see all the way from east to west) (animation by G. Rieke); you may have to reload to start it.|
|and here is how they would look from the North Pole (assuming our super-wide-angle eyes take in the pole star at the top of the animation) (by G. Rieke).|
It is this behavior that suggested that the stars and planets were placed on great crystal spheres that were centered on the earth and rotated around it.
|The dependence of apparent positions on the sky on north-south position resulted in using stars for navigation.||Reconstructed Greek Trireme, Olympios, (from Perseus Encyclopedia, http://www.perseus.tufts.edu/cgi-bin/image?lookup=1989.02.0009)|
|"Glorious Odysseus, happy with the wind, spread
sails and taking his seat artfully with the steering oar he held her on her course ['her'
is a raft he built to sail away from the nymph Kalypso], nor did sleep ever descend on his
eyelids as he kept his eye on the Pleiades [a cluster of stars] and late-setting Bootes [a
constellation, an identifiable pattern of stars in the sky] and the Bear [the
constellation now known as the Big Dipper], to whom men give also the name of the Wagon,
who turns about in a fixed place and looks at Orion [another easily identified
constellation], and she alone is never plunged in the wash of the Ocean. For so Kalypso,
bright among goddesses, had told him to make his way over the sea, keeping the Bear on his
-- Homer, the Odyssey, from the translation by Richmond Lattimore, Perrenial Classics edition, 1999, HarperCollins, publishers.
|There was always a problem with east-west positions, though, because
they required accurate time keeping
Phoenician ship (far left); Phoenician navigation route (left): shoreline was only reliable east-west guide (from Geocities, http://www.geocities.com/SoHo/Cafe/9535/navigate.htm)
Some objects appeared to be points of light that move with respect to the stars. The Greeks gave this type of object the name "planet" for wanderer. Mercury, Venus, Mars, Jupiter, and Saturn have been known since pre-historic times.
The sun and the moon always appear to move eastward against the pattern of stars, and the planets also generally move eastward, but once in awhile, a planet will appear to turn around and move westward for awhile before returning to a generally eastward motion. This "retrograde motion" posed an intellectual puzzle that occupied astronomers for thousands of years! In fact, the motions on the sky were so dramatic and precise that we can truly claim that trying to explain them was responsible for the beginnings of precise scientific theory for all fields.
|Retrograde motions of planets trace onto the sky as shown to the left. As an ancient astronomer observed Mars over a period of about seven months, it would have traced a large loop against the background of stars. For a demonstration, see http://alpha.lasalle.edu/~smithsc/Astronomy/retrograd.html, part of which is reproduced below.|
|Motion of Mars, converted to a movie of apparent motion. Compare the run of dates in the lower right of the animation. The animation shows the total motion of Mars against the stars over a period of about 9 months.|
These complications led to sophisticated systems of astronomy/astrology. To predict dramatic celestial events, the astronomers/priests/astrologers developed accurate long term calendars and texts.
The Maya of meso-America provide an example of great accomplishments in astronomy, which they embodied into religious/ceremonial aspects of their culture.
|For example, the El Castillo pyramid, Chichen Itza, Yucatan, Mexico, played the role, among other things, of marking the spring equinox in a public and dramatic fashion. To the left is a picture of El Castillo at the time of the spring equinox. The shadows trace out an immense snake slithering down the side of the pyramid (this happens only on this special day). Such dramatic demonstrations had more to do with impressing the public with the power of the priests than low-budget astronomy, but they did require accurate observations and major efforts to understand them. (http://www.rose-hulman.edu/~delacova/chichen-00.htm)|
|Building such large monuments therefore required substantial investments in astronomy as well as in architecture An example is El Caracol, a Mayan observatory at Chichen Itza, not far from El Castillo (G. Rieke)|
|Temples at Tikal, Guatemala, are elevated above the treetops to allow unobstructed viewing of the stars. Temple IV at Tikal, built in about 470AD, rises to a height of 212 feet above the jungle floor. (G. Rieke)|
The Maya achieved impressive accuracy in measuring the repetitions of celestial events (synodic periods are the times required for the body to come back to the same position relative to the sun):
|Modern (days)||Maya in days|
|Lunar (synodic) month||29.53059||29.53086|
|Synodic period of Venus||583.93||583.92027|
|Synodic period of Mars||779.94||780|
|Solar (tropical) year||365.24198||365.242|
However, this accomplishment was addressed more toward prediction of propitious times for rulers to take action, that is, predicting the future. It belongs more in the realm of a very sophisticated astrology than to that of astronomy. The failure of the Mayas to seek underlying causes for the processes they measured so precisely meant that their approach was a dead end scientifically.
|Where it is a duty to worship the sun it is pretty sure to be a crime to
examine the laws of heat.
- Viscount Morley John, Voltaire
Not all ancient societies were as accomplished in astronomy as the Maya. However, virtually all had myths associated with the sky and earth, particularly creation myths In general, such myths are not refined through detailed measurements (such as those in the table above by the Maya) and they may not even be updated as new astronomical knowledge is acquired. Thus, they are basically not examples of a scientific approach.
|Star petroglyphs, in "Planetarium Cave," Canyon de Chelly, from Gary Tepfer, http://www.wlotus.com/GaryTepfer/default.htm||
Urania, Greek muse of astronomy, by Simon Vouet, http://www.luminarium.org/sevenlit/james/urania.htm
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hypertext G. H. Rieke
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